RESEARCH ARTICLE Open Access

Mastoid notch as a landmark forlocalization of the transverse-sigmoid sinusjunctionRuichun Li, Lei Qi, Xiao Yu, Kuo Li and Gang Bao*

Abstract

Background: The top of the mastoid notch (TMN) is close to the transverse-sigmoid sinus junction. The spatialposition relationship between the TMN and the key points (the anterosuperior and inferomedial points of thetransverse-sigmoid sinus junction, ASTS and IMTS) can be used as a novel method to precisely locate the sinusjunction during lateral skull base craniotomy.

Methods: Forty-three dried adult skull samples (21 from males and 22 from females) were included in the study. Arectangular coordinate system on the lateral surface of the skull was defined to assist the analysis. According to sexand skull side, the data were divided into 4 groups: male&left, male&right, female&left and female&right. Thedistances from the ASTS and IMTS to the TMN were evaluated on the X-axis and Y-axis, symbolized as ASTS&TMN_x, ASTS&TMN_y, IMTS&TMN_x and IMTS&TMN_y.

Results: Among the four groups, there was no significant difference in ASTS&TMN_x (p = 0.05) and ASTS&TMN_y(p = 0.3059), but there were significant differences in IMTS&TMN_x (p < 0.001) and IMTS&TMN_y (p = 0.01), andmultiple comparisons indicated that there were significant differences between male&left and female&left both inIMTS&TMN_x (p = 0.0006) and in IMTS&TMN_y (p = 0.0081). In general, the ASTS was located 1.92 mm anterior tothe TMN on the X-axis and 27.01 mm superior to the TMN on the Y-axis. For the male skulls, the IMTS was located3.60 mm posterior to the TMN on the X-axis and 14.40 mm superior to the TMN on the Y-axis; for the female skulls,the IMTS was located 7.84 mm posterior to the TMN on the X-axis and 19.70 mm superior to the TMN on the Y-axis.

Conclusions: The TMN is a useful landmark for accurately locating the ASTS and IMTS.

Keywords: Transverse sinus, Sigmoid sinus, Mastoid notch, Skull base

BackgroundThe anterosuperior and inferomedial points of thetransverse-sigmoid sinus junction (ASTS and IMTS)represent the most posterior edge of the middle fossaand the most superolateral limit of the retrosigmoidapproach, respectively. Accurately locating these keypoints on the external surface of the cranium is import-ant in lateral skull base craniectomy [1–4]. Traditionally

the squamosal-parietomastoid suture junction (SP) andasterion have been regarded as the classic landmarks forassisting with identifying the ASTS and IMTS, respect-ively [5–7]. However, the cranial sutures is covered byperiosteum, and this condition often leads to the inabil-ity to recongnize the SP and asterion [7, 8]. Therefore, itis necessary to find a more reliable and practical methodto precisely locate the key points.The mastoid notch is a deep groove on the medial side

of the mastoid process that can be clearly identified.Because the top of the mastoid notch (TMN) is close to

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* Correspondence: baogang1973@xjtu.edu.cnDepartment of Neurosurgery, First Affiliated Hospital of Xi’an JiaotongUniversity, 277 West Yanta Road, Xi’an 710061, Shaanxi, China

Li et al. BMC Neurology (2020) 20:111 https://doi.org/10.1186/s12883-020-01688-2

the transverse-sigmoid sinus junction, it is possible tomake it a potential landmark for locating the ASTS andIMTS [9, 10].The purpose of this study was to analyze the spatial

position relationship between the TMN and the ASTSand IMTS and to provide a new method for the accuratepositioning of these key points in lateral skull basecraniectomy.

MethodsThis study was approved by the Ethics Committee of theFirst Affiliated Hospital of the Medical College of Xi’anJiaotong University (KYLLSL-2014-129-01).

Skull samplesForty-three dried adult skulls (age ≥ 18 years, 86 sides)were provided by the Department of Anatomy at theMedical College of Xi’an Jiaotong University. The bonysulci of the transverse and sigmoid sinuses as well asthe mastoid notch were clearly discernible. Of theskulls, 21 were from males, and 22 were from females.The skull circumferences were measured through inter-cilium anteriorly and the external occipital protuber-ance posteriorly.

Definations of the ASTS, IMTS and TMNThe transverse-sigmoid sinus junction was defined asthe transitional zone where the transverse sinus endedby the vertical descending segment of the sigmoid sinus;the ASTS and IMTS were identified as the anterosuper-ior and inferomedial points of the junction, respectively[8, 11] (Fig. 1). The projection of the two key points to

the outer surface of the skulls was marked with acolored pencil. The TMN was defined as the mostsuperomedial point of the notch (Fig. 2).

Reference coordinate systemA reference rectangular coordinate system was devisedto help analyze the spatial position relationship betweenthe TMN and the ASTS and IMTS on the externalsurface of the skull. The X-axis was defined by points Aand B. Point A was located where the upper edge of thezygomatic arch (UEZA) joins anteriorly with the frontalprocess of the zygomatic bone (FPZ), and point B waslocated where the UEZA blends posteriorly into thesupramastoid crest (SMC). The Y-axis was a straight linethat passed through the tip of the mastoid (point C) andwas perpendicular to the X-axis. On the X-axis, theposterior side was positive, and the anterior side wasnegative. On the Y-axis, the superior side was positive,and the inferior side was negative (Fig. 3).The ASTS, IMTS and TMN were vertically projected

onto the axes. ASTS_x, IMTS_x and TMN_x representthe coordinates of the ASTS, IMTS and TMN on the X-axis, respectively; ASTS_y, IMTS_y and TMN_y repre-sent the coordinates of those points on the Y-axis,respectively.The distances from the ASTS to the TMN were calcu-

lated on the X-axis and Y-axis separately and weredenoted by ASTS&TMN_x and ASTS&TMN_y(ASTS&TMN_x = ASTS_x – TMN_x, ASTS&TMN_y =ASTS_y – TMN_y). Similarly, the distances from theIMTS to the TMN were denoted by IMTS&TMN_x and

Fig. 1 Inner surface of the cranium. The green and yellow circles represent the sites of the ASTS and IMTS, respectively. The purple translucentarea represents the transverse-sigmoid sinus junction. TS: transverse sinus; SS: sigmoid sinus; JF: jugular foramen

Li et al. BMC Neurology (2020) 20:111 Page 2 of 7

Fig. 2 Outer surface of the cranial base. On the medial side of the mastoid process, the mastoid notch extends superomedially to end up as thetop of the notch. MP: mastoid process; MN: mastoid notch; TMN: top of the mastoid notch; EAM: external auditory meatus

Fig. 3 Illustration of the coordinate system on the left side of a skull sample. The X-axis is established by the horizontal line connecting points Aand B, which are located where the upper edge of the ZA joins anteriorly to the FPZ and blends posteriorly into the SMC, respectively. The Y-axisis defined by a line through the tip of the mastoid (Point C) and perpendicular to the X-axis. The red, yellow and green circles represent the TMN,IMTS and ASTS, respectively. FPZ: frontal process of the zygomatic bone; ZA: zygomatic arch; SMC: supramastoid crest

Li et al. BMC Neurology (2020) 20:111 Page 3 of 7

IMTS&TMN_y (IMTS&TMN_x = IMTS_x – TMN_x,IMTS&TMN_y = IMTS_y – TMN_y) (Fig. 4).

Statistical analysesTwo researchers in this study measured the values of theASTS, IMTS and TMN in the coordinate system using aVernier caliper with anaccuracy of 0.02 mm. Each sam-ple was measured three times, and the average value wastaken as the result. According to sex and skull side, thedata were divided into 4 groups: male&left, male&right,female&left and female&right. A normality test wasperformed using the Shapiro-Wilk test (α = 0.1), and thehomogeneity of variance test (α = 0.1) was conducted bythe Levene test. When the variables conformed to a nor-mal distribution, the mean ± standard deviation was usedfor statistical description; otherwise, the median wasused.To estimate the group size, a pilot study was con-

ducted to measure ASTS&TMN_x, ASTS&TMN_y,IMTS&TMN_x, and IMTS&TMN_y in 20 skulls. With atwo-tailed α = 0.05 and a power of 80%, the studyrequired 31 skulls. Ultimately, 43 skull specimens weremeasured in this study.Because not all the variables conformed to the normal

distribution, Dunn’s test with Bonferroni adjustment ofthe p-value was used for multigroup rank sum test.When the differences in the average levels among groupswere statistically significant, the multiple comparisonswere carried out. A two-tailed p < 0.05 indicated statis-tical significance. All statistics were performed with R

version 3.5.0 (Copyright (C) 2018 The R Foundation forStatistical Computing).

ResultsSkull circumference and key point coordinatesThe average male and female skull circumferences were49.88 ± 1.19 cm and 48.93 ± 1.13 cm, respectively. Themedians of the ASTS, IMTS and TMN coordinates arepresented in Table 1.The relative coordinates between the ASTS and IMTS

and the TMN are shown in Table 2.

ASTS and TMNThe ASTS was located 1.92 mm anterior to the TMN onthe X-axis and 27.01 mm superior to the TMN on theY-axis (see Table 2 for details). There were no significantdifferences among the 4 groups in ASTS&TMN_x (p =0.05) or ASTS&TMN_y (p = 0.3059) (see Table 3 fordetails).

Fig. 4 The distances from the IMTS to the TMN were analyzed in the X- and Y-axes directions, symbolized as IMTS&TMN_x and IMTS&TMN_y,respectively. So did that from the ASTS to the TMN

Table 1 Coordinates of the ASTS, IMTS and TMN

ASTS_x ASTS_y IMTS_x IMTS_y TMN_x TMN_y

median, mm

ML 16.48 7.45 20.78 −4.26 18.36 −18.36

MR 16.32 8.01 25.18 −4.78 19.76 − 19.84

FL 15.23 7.26 21.77 −1.60 16.00 −20.60

FR 15.41 7.44 24.20 −4.43 16.71 −21.00

M male, F female, L left side, R right side

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IMTS and TMNThere were significant differences among the 4 groupsin IMTS&TMN_x (p < 0.001) and IMTS&TMN_y (p =0.01) (see Table 3 for details). Furthermore, multiplecomparisons indicated that there were significant differ-ences between male&left and female&left both inIMTS&TMN_x (2.56 mm vs 8.38 mm, p = 0.0006) and inIMTS&TMN_y (14.32mm vs 18.69 mm, p = 0.0081).Then, the medians of the IMTS&TMN_x andIMTS&TMN_y were calculated according to sex (seeTable 4 for details). For the male skulls, the IMTS was3.60 mm posterior to the TMN on the X-axis and 14.40mm superior to the TMN on the Y-axis; for the femaleskulls, the IMTS was 7.84 mm posterior to the TMN onthe X-axis and 19.70 mm superior to the TMN on theY-axis.

DiscussionOver the past decades, neurosurgeons have been usinginnovative techniques to safely expose the transverseand sigmoid sinuses without resulting in extensive bonydefects during craniotomy. The accurate placement ofthe keyhole at the transverse-sigmoid sinus junction isone of the most important steps in this procedure [11–14]. Goto T used the lateral end of the transverse sinus,the ASTS, the mastoid emissary foramen and the mid-point of the transverse sinus as the site of 4 key holes tocomplete the exposure of the sigmoid sinus [2]. To avoidextensive bony defects in the periauricular area, Jia rec-ommended a two-bone flap craniotomy technique forthe transpetrosal presigmoid approach; this technique

required that the first bone flap distinctly expose theASTS to facilitate the dissection of the sigmoid sinusaway from the inner table of the mastoid bone [15]. Usu-ally, the squamosal-parietomastoid suture junction (SP)and the intersection of the supramastoid crest with thesquamosal suture (SCSS) are accepted as importantlandmarks for representing the ASTS [5, 7, 15, 16]. Un-fortunately, the squamosal and parietomastoid suturesare often difficult to recognize during craniotomy, espe-cially in older adults. Therefore, the SP and the intersec-tion of the SCSS are not reliable landmarks for theASTS [8, 17].The asterion is located at the junction of the parieto-

mastoid, lambdoid and occipitomastoid sutures. It wasonce considered a classic bony landmark for the IMTS[18, 19]. However, there is growing evidence that placingkey holes in the asterion may cause unexpected sinusdamage [8, 20–22]. Teranishi investigated the distancefrom the transverse-sigmoid sinus junction to the aster-ion by using three-dimensional computed tomographyimages, and his study indicated that the keyhole shouldbe placed 6.5 mm both laterally and caudally to theasterion [21]. Moreover, the asterion is often difficult torecognize because it is sometimes difficult to identify theparietomastoid, lambdoid and occipitomastoid sutures.Therefore, the asterion it is not a reliable landmark forlocating the IMTS [8, 23, 24].Image-guided surgical planning, including neuronavi-

gation and other methods based on the 3D volume ren-dering (3D VR) technique, can yield morphometric datain individual patients and can overcome extreme indi-vidual variations [7, 11, 25, 26]. For instance, Xia utilizeda line connecting the digastric point and the asterion toestablish a coordinates systemon for 3D VR images; thenhe analyzed the coordinate relationship between theasterion and the IMTS to assist with localizing the IMTS[22]. However, these techniques are both expensive and

Table 2 Relative coordinates between key points and TMN

ASTS&TMN_x ASTS&TMN_y IMTS&TMN_x IMTS&TMN_y

median, mm

ML −2.84 26.23 2.56 14.32

MR −3.16 27.47 4.44 14.48

FL 0.19 27.66 8.38 18.69

FR −1.40 27.15 7.57 19.74

overall −1.92 27.01 5.46 15.95

M male, F female, L left side, R right side. For the x relative coordinates, sign "-"means the key point was located on the frontal side of the TMN; For the yrelative coordinates, −"means the key point was located on the inferior side ofthe TMN

Table 3 Comparisons between genders and sides for ASTS&TMN and IMTS&TMN

ASTS&TMN_x(p = 0.05)

ASTS&TMN_y(p = 0.3059)

IMTS&TMN_x(p < 0.001)

IMTS&TMN_y(p = 0.01)

ML vs MR p= 1.000 0.4821 0.1581 1.000

FL vs FR 0.5404 1.000 1.000 1.000

ML vs FL 0.1340 0.2559 0.0006* 0.0081*

MR vs FR 0.5173 1.000 0.075 0.5603

Dunn test with Bonferroni adjustment of the p-value was used for multi-group rank sum test; * p < 0.05

Table 4 IMTS&TMN_x and IMTS&TMN_y of the male and female

IMTS&TMN_x IMTS&TMN_y

median, mm

Male 3.60 14.40

Female 7.84 19.70

Li et al. BMC Neurology (2020) 20:111 Page 5 of 7

time-consuming. Moreover, they may be limited byequipment problems, emergency cases and allergies tocontrast medium [8, 22].In summary, the methods mentioned above have the

following shortcomings: 1) skull sutures sometimes cannot be identified distinctly, which makes the positioningineffective;, 2) traditional landmarks, such as the aster-ion, are not precise enough to locate the key point; and3) image-guided surgical planning is both expensive andtime-consuming. Therefore, it is still worth identifyingan accurate, fast, practical and low-cost method to locatethe sinuses.Tubbs introduced a reference coordinate system estab-

lished by the X-axis extending along the most superiorborder of the zygomatic arch and the Y-axis extendingfrom the mastoid notch to the squamosal suture [8]. In100 adult skulls (200 sides), the distances from the IMTSto the X- and Y-axes were measured and analyzed statis-tically, and the results were used to locate the IMTS inthe retrosigmoid approach. This method does not re-quire the identification of any skull sutures. Over thepast few years, the method has been modified, but inpractice, it is difficult to precisely define the X- and Y-axes; slight coordinate translation or coordinate rotationcan lead to major errors in the locating system [20, 24].In the present study, points A, B and C could be easily

identified during craniotomy [3, 8, 20, 24]. However, themost important part of this study was that we utilized areference point, the TMN, to improve the accuracy oflocalization. Because this method is based on the relativeposition between the TMN and the ASTS or IMTS, it isnot affected by coordinate translation. In other words,the relative coordinates remain constant when the co-ordinate system is translated. Therefore, this newlocalization method should be more accurate and prac-tical than before [8, 20, 24].

Application of this studyBefore disinfection of the operation area, a colored penshould be used to outline the X- and Y-axes on thescalp. After exposing the bony surface of the skull base,the axes should be marked on the bone surface by uni-polar electrocoagulation, and the TMN point can befound at the same time.

ASTS and TMNBecause there was no significant difference in sex orskull side, the ASTS can be located 1.92 mm anterior toand 27.01 mm superior to the TMN with directionsparallel to the X- and Y-axes, respectively.

IMTS and TMNThere were significant differences between male andfemale skulls both in IMTS&TMN_x and in

IMTS&TMN_y, which indicate that the distance fromthe IMTS to the TMN in males was shorter than that infemales. Then, for the male skulls, the IMTS can belocated 3.60 mm posterior to and 14.40 mm superior tothe TMN; for the female skulls, the IMTS can be located7.84 mm posterior to and 19.70 mm superior to theTMN. Additionally, the directions should be parallel tothe X- and Y-axes.

Limitation of the studySince the data in this study were obtained from 43 adultskulls, the results should not be applied to children.Additionally, there was occasionally a high degree of in-dividual variation in the relationship between the TMNand the key points, which thus increases the risk of sinusinjury [25, 26]. Therefore when conditions permit, neu-ronavigation and other image-assisted positioning tech-nologies should be taken into account to overcomeextreme variations.

ConclusionsThe TMN is a useful landmark for accurately locatingthe ASTS and IMTS. However, The results of this studyare only applicable to adult cases.

AbbreviationsASTS: The anterosuperior point of the transverse-sigmoid sinus junction;FPZ: The frontal process of the zygomatic bone; IMTS: The inferomedial pointof the transverse-sigmoid sinus junction; JF: Jugular foramen; MN: Mastoidnotch; MP: Mastoid process; SMC: Supramastoid crest; TMN: The top of themastoid notch; UEZA: Upper edge of the zygomatic arch

AcknowledgementsWe thank Prof. Weixi Wang, the Department of Anatomy of Medical Collegeof Xi’an Jiaotong University, for providing specimens for this study.

Authors’ contributionsGB conceived and designed the study. Dry skull specimens preparation, datacollection and analysis were performed by RL, LQ, KL and XY. The first draftof the manuscript was written by RL, and all the authors commented onprevious versions of the manuscript. All the authors read and approved thefinal manuscript.

FundingThis study was funded by a grant from Key Research and Development Planof Shaanxi Province, China (No. 2018SF-137). The sponsor had no role in thedesign or conduct of this research.

Availability of data and materialsAll data generated or analysed during this study are included in thispublished article.

Ethics approval and consent to participateThis study was approved by the Ethics Committee of the First AffiliatedHospital of the Medical College of Xi’an Jiaotong University (KYLLSL-2014-129-01).

Consent for publicationNot applicable.

Competing interestsThe authors declare that they have no competing interests.

Li et al. BMC Neurology (2020) 20:111 Page 6 of 7

Received: 21 November 2019 Accepted: 16 March 2020

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